When a structure to be built up from such prefabricated members is long enough to extend across three or more supporting piers, their emplacement generally requires the use of a bridge crane which can be moved horizontally across these piers and also carries hoist means for raising the precast beams or slabs off an adjoining landing and depositing them in proper position on the piers. The equipment heretofore employed for this purpose has been relatively cumbersome and not very reliable in operation, particularly in those instances in which a single control mechanism is utilized for the horizontal crane motion and for the operation of the hoist. Even with separate driving and hoisting mechanisms the performance has not always been satisfactory.